Active matrix display with a scanning backlight

ABSTRACT

The invention relates to an active matrix display system and a method of operating such a system. The system comprises an active matrix display panel and a scanning backlight including several light members ( 3   a - e ) each of which illuminates a pixel line or a sector of pixel lines, in a fully activated state ( 8 ) of high light intensity for a limited period of time when the pixel lines in that sector have been addressed ( 6   a - e ) according to a video signal and have reached a new fully or nearly fully modulated state. This is done to eliminate or reduce the tendency of the so called “Sample and Hold” artefact when displaying fast moving objects on the display. According to the invention the light members ( 3   a - e ) are not switched off completely during a time period between two successive fully activated states of high light intensity. Instead they are operated to a reduced, dimmed or glowing state ( 9 ) with low light intensity of 10-50%, preferably 15-40% and most preferred 20-30% of the high light intensity in the fully activated state to enhance brightness and reduce flicker of the display.

The present invention relates to an active matrix display system with ascanning backlight, for example an active matrix LCD-system for use in aTV set or in a monitor.

The invention also relates to a method of operating such an activematrix display system.

One of the most important requirements a display has to meet in order tobe used for video applications is to provide sharp motion pictures.Nevertheless, motion pictures shown on an active matrix display, e.g. anactive matrix LCD (Liquid Crystal Display) panel, loose their sharpnessand become blurred when displaying fast moving objects even when veryfast LCD panels are used. The reason for this is that the motion blur isnot caused by the speed of the pixels (picture elements) only. It isalso caused by the combination of the hold characteristics of thedisplay since each line of pixels is modulated once in a modulatingcycle or frame time and thereafter transmits light continuously from abacklight until the next modulating cycle, and the characteristics ofthe human eye trying to follow the motion. This effect is known as the“Sample and Hold” artefact and it is not limited to active matrixLC-displays only. Every active matrix display suffers from the sameproblem.

The most efficient method for removing the “Sample and Hold” artefact inactive matrix display panels is the use of so called scanning backlight,i.e. displays which are operated not to emit light continuously but inshort time pulses. The pulsed light exposure of each pixel should becarried out once in a frame time and not before the pixel has reachedits desired transmission level, i.e. when the pixel has been addressedand has reached a new fully or near fully modulated state. Because theactive matrix display panel is addressed line by line, this moment intime is different for every line. Therefore, it is necessary to usescanning backlight which is synchronized with the video signal. Suchbacklight produces a horizontal band of light of a certain width thatscans vertically over the panel. Due to the stroboscopic effect of thebacklight, the object is seen only at the right moments when the pixellines in question just have been modulated, which yields a sharpperceived image.

The method according to scanning backlight does not demand fast pixelresponse and it enables removing of the motion blur artefacteffectively. It enables a perfect motion portrayal with a pixel responsetime that lies just within a frame time (˜16 ms for 60 Hz frame rate and˜20 ms for 50 Hz frame rate). No extensive signal processing is requiredeither.

There are several other methods known in the art to remove or at leastreduce the “Sample and Hold” artefact, but scanning backlight has provedto be the only method which is able to remove the “Sample and Hold”effect completely. Also, the dynamic contrast and colour purity areincreased when this method is used, which makes the use of scanningbacklight the most beneficial method for removing the “Sample and Hold”artefact.

However, scanning backlight has also some disadvantages. The mostimportant one is the loss of brightness. Decreasing the exposure time ofthe pixels decreases the amount of the light which they emit, making thepicture darker. The brightness decrease is proportional to the dutycycle of the backlight, i.e. the time ratio between “on” and “off”states of each pixel.

Another disadvantage of the scanning backlight method is appearance offlicker. Due to the stroboscopic nature of the backlight, observer hasan impression that the whole display blinks with the frequency of theframe rate.

As an example of a prior art scanning backlight device and method,reference is made to WO 9501701 A.

It is an object of the invention to provide an active matrix displaysystem with an improved scanning backlight. More precisely it is anobject to enhance the brightness and remove the tendency of flicker foran active matrix display system provided with a scanning backlight. Atleast this object is achieved by an active matrix display systemaccording to claim 1.

The invention also relates to a method of operating an active matrixdisplay system provided with a scanning backlight, with essentially thesame object as above. This object is achieved by a method according toclaim 5.

The invention is thus based on the understanding that the brightness maybe enhanced and the tendency of flicker may be removed and yet yield asharp perceived image with removed or at least reduced “Sample and Hold”artefact, by driving the scanning backlight such that the light is notcompletely switched off in the time period between two modulatingoperations of the lines of pixels. Instead the backlight is caused to bedimmed or glow with low intensity in the time period between twomodulating operations of the pixels. Since the brightness is a functionof the average light intensity during a time period, it is evident thatthe brightness is increased if the lowest light intensity level isincreased from 0% to between 10-50%, preferably to between 15-40% andmost preferred to between 20-30% of the maximum light intensity. Also,the flicker is reduced as the difference between the maximum and theminimum light intensity is decreased. Yet, the low intensity back lightis reduced to such an extent that the image is not deteriorated sincedetails of the picture is not actually perceived during the period withlow light intensity. Accordingly, the display is able to provide sharpmotion pictures without any “Sample and Hold” artefact.

In an active matrix display provided with a scanning backlight accordingto the invention, each line of pixels may be illuminated by a separatelight member. However, since e.g. an active matrix LCD-display normallycontains more than 1000 lines, such a solution would result in acomplicated and hence expensive display. Usually, an acceptablyperformance of the display, may be achieved by providing a light memberfor e.g. every 50 to 150 lines of pixels and consequently normally about7 to 12 light members for each display.

The light members may be operated in sequence, so that only one lightmember is in its fully activated state with high light intensity at eachtime. I.e. one light member is reduced from its high light intensity toits reduced or dimmed state with low light intensity simultaneously withactivating the subsequent light member to its high light intensitystate. Normally however, the light members are activated with someoverlap so that two or more light members are in their fully activatedstates of high light intensity simultaneously.

All translucent active matrix displays, i.e. displays with a backlight,can use the scanning backlight according to'the present invention forremoving the motion artefacts, and the described improvement of thescanning backlight driving method, according to the present invention,can be applied to all systems where a scanning backlight system isapplied.

Furthermore, with a scanning backlight according to the invention, thereis electric current flowing through the light members all the time.Therefore it is not necessary to re-ignite the light members in thebacklight every time their fully activated period comes. This makesdriving of the light members much easier and construction of invertersfor their driving simpler. The fact that re-ignition of the lightmembers is not necessary at the beginning of every fully activatedperiod, increases the life time of the light members in comparison tothe ordinary backlight. When the light members are completely switchedoff during the period between two consecutive fully activated periods,like in ordinary scanning backlight, their temperature is lower than theoptimal working temperature because they cool down during theinactivated period when they are switched off. With the proposed drivingmethod the temperature of the light members is higher due to the currentwhich constantly flows through them, i.e. the temperature is closer tothe optimum working temperature.

The present invention involves a compromise between the increase of thedisplay light output and the efficiency of motion artefact suppression.Nevertheless, experiments show that it is possible to introduce theproposed bias of the light members to a minimum light intensity andachieve substantial brightness increase without a noticeable degradationof motion picture quality.

These and other aspects of the invention will be apparent from andelucidated with reference to the embodiment described hereinafter.

The invention will now be explained by way of example with reference tothe accompanying drawings in which:

FIG. 1 is a diagrammatic exploded perspective view of an active matrixdisplay panel and a scanning backlight with the uppermost light memberin a fully activated state of high light intensity;

FIG. 2 is a view similar to FIG. 1 but with the second uppermost lightmember in a fully activated state; and

FIG. 3 is a timing diagram illustrating the sequential addressing of thelines of pixels and the activating rate of the light members,respectively, in time sequence.

In the drawing is shown a preferred embodiment of an active matrixdisplay panel 1, e.g. an active matrix LCD-display panel, provided witha scanning backlight 2. The display panel includes a large number ofpixels arranged in normally more than one thousand horizontal lines ofpixels. Each pixel may be continuously variable in a desired degree fromsubstantially a none-translucent to a maximum translucent state. This isdone electrically by addressing the pixels line by line several timesper second, normally 50 or 60 times per second. When the pixels are in atranslucent state, they are capable of transmitting light from thescanning backlight to a desired extent which creates an image that aviewer in front of the display can see. By changing the image in smallsteps and short time intervals, it is created an illusion of acontinuously moving picture.

The scanning backlight 2 in the figures comprises, for the sake ofsimplicity, five separate light members 3 a-e, but it should beunderstood that many more light members may form part of a displaysystem according to the invention. Each light member is adapted toilluminate a horizontal sector 4 a-e, including several pixel lines,from the back of the display panel. This is not done until every pixelin a sector has been modulated by an addressing signal and each pixelhas reached its fully or near fully modulated state.

According to the principle of scanning backlight, the light members 3a-e will in sequence illuminate their allocated sector from the back ofthe display panel. Thus, in FIG. 1, the uppermost light member 3 a isfully activated at high light intensity and illuminates an uppermostsector 4 a of the display panel, in which each pixel has been modulatedto the desired translucent state. When all the pixels in the subsequentsector 4 b of the display panel has been addressed and modulated, theuppermost light member is brought into a reduced state of low lightintensity while, slightly before that, the second uppermost light member3 b is fully activated to illuminate the back of the second uppermostsector of the display panel, as is illustrated in FIG. 2. In the sameway, the rest of the display panel is illuminated, sector after sector,until the whole display panel has been illuminated in a frame time,whereafter the process starts all over again with the uppermost sectorand light member.

The process is illustrated in the timing diagram of FIG. 3, in which 5a-e discloses the variation of the light intensity between the reducedand the fully activated state of the light members 3 a-e , and 6 a-ediscloses the variation of the modulation rates of the pixels in eachsector 4 a-e at each point of time during the frame cycle. As mentionedbefore, each pixel line is addressed in sequence at different points oftime. Since each sector 4 a-e contains several pixel lines, this meansthat the pixel lines in each sector are addressed at different points oftime as well. Consequently, the graphs 6 a-e are illustrations of theaddressing and modulating of the last pixel line in each sector, whereasthe rest of the pixel lines in the same sector are somewhat ahead inrespect of time.

Each sector, that is to say the last pixel line in each sector, isaddressed at a point of time marked by an arrow 7, by an addressingsignal from an addressing system according to a video signal. The graphs6 a-e illustrate how the pixels in each sector goes from an unchanged ornone modulated state in relation to a preceding addressing andmodulation, to a new fully modulated state. This modulating time mayvary in dependence of how fast the pixels are. When the pixels in onesector has reached, or is close to, a fully modulated state, the lightmember for that specific sector is fully activated to shine with highlight intensity, as illustrated by an upward directed rectangular block8.

The pixels remain in the same modulated state until the subsequentaddressing signal. As illustrated in the drawing, a fully activatedlight member is brought to the reduced state with a somewhat overlap inrespect of the subsequent light member when the last pixel line in thatsector has reached a fully or nearly fully modulated state, i.e. twoadjacent light members are fully activated simultaneously for a shortperiod of time.

However, according to the invention, when the light members are reduced,they are not switched off completely. Instead their light intensity isreduced to a dimmed or glowing state with a light intensity that is10-50%, preferably 15-40% and most preferably 20-30% of the high lightintensity in the fully activated state. This is illustrated in FIG. 3 bya horizontally extended strip 9 which defines a low light intensity ofthe light members in the reduced state in the period between twosuccessive high light intensity periods 8. The numeral 10 in FIG. 3denotes a frame time between the addressing of the first and last pixelline of the display.

In summary the invention relates to an active matrix display system anda method of operating such a system. The system comprises an activematrix display panel and a scanning backlight including several lightmembers 3 a-e, each of which illuminates a pixel line or a sector ofpixel lines, in a fully activated state 8 of high light intensity for alimited period of time when the pixel lines in that sector have beenaddressed 6 a-e according to a video signal and have reached a new fullyor nearly fully modulated state. This is done to eliminate or reduce thetendency of the so called “Sample and Hold” artefact when displayingfast moving objects on the display. According to the invention the lightmembers 3 a-e are not switched off completely during a time periodbetween two successive fully activated states of high light intensity.Instead they are operated to a reduced, dimmed or glowing state 9 withlow light intensity of 10-50%, preferably 15-40% and most preferred20-30% of the high light intensity in the fully activated state toenhance brightness and reduce flicker of the display.

1. An active matrix display system comprising a display panel havinglines of picture elements for modulating light, which are separatelydrivable in a continuously variable manner between a maximum translucentstate and a substantially non-translucent state, several light membersfor illuminating the display panel from a rear side to produce a displayoutput in accordance with the degree of translucency of each pictureelement, an addressing system for addressing the lines of pictureelements sequentially to a desired translucent state of each pictureelement according to a video signal, and a driving system for drivingeach light member in a fully activated state of high light intensity fora limited period of time to selectively illuminate one or more lines ofpicture elements at a time, synchronous with the addressing system, wheneach pixel has reached its fully or near fully modulated state,characterised in that the light members Fin the time period between twoconsecutive fully activated states, are operated to a reduced state oflow light intensity.
 2. An active matrix system according to claim 1,characterised in that the light intensity of the light members in thereduced state is between 10% and 50% of the light intensity in the fullyactivated state.
 3. An active matrix system according to claim 1,characterised in that the light intensity of the light members in thereduced state is between 15% and 40% of the light intensity in the fullyactivated state.
 4. An active matrix system according to claim 1,characterised in that the light intensity of the light members in thereduced state is between 20% and 30% of the light intensity in the fullyactivated state.
 5. A method of operating an active matrix displaysystem, comprising a display panel having lines of picture elements formodulating light, several light members on a rear side of the panel, anaddressing system for addressing the picture elements, and a drivingsystem for driving the light members in a fully activated state of highlight intensity, the method comprising the steps of modulating thepicture elements by means of the addressing system in a continuouslyvariable manner to a desired degree between a maximum translucent stateand a substantially non-translucent state by modulating the pictureelements sequentially line after line in accordance with a video signal,to selectively illuminate one or more lines of picture elements at atime in the fully activated state for a limited period of time, from therear side of the display panel, synchronous with the addressing system,to produce a display output in accordance with the degree oftranslucency of each picture element, characterised by the further stepto drive the light members (3 a-e) in a reduced state of low lightintensity during a time period between two consecutive fully activatedstates.
 6. A method according to claim 5, characterised by driving thelight members such that the light intensity in the reduced state isbetween 10% and 50% of the light intensity in the fully activated state.7. A method according to claim 5, characterised by driving the lightmembers such that the light intensity in the reduced state is between15% and 40% of the light intensity in the fully activated state.
 8. Amethod according to claim 5, characterised by driving the light memberssuch that the light intensity in the reduced state is between20% and 30%of the light intensity in the fully activated state.